"Breeding-back" aims to restore or immitate extinct animals by selective breeding. This blog provides general information, the facts behind myths and news from various projects.

Tuesday, 1 August 2017

The genetic and developmental background of visible traits

I have been
planning to do such a post for quite some
time now, and have gathering information for several months. Finally I had the
time to put it all together. I think it is a really fascinating subject that
can tell us a lot about how to achieve the traits we want our “breeding-back”
cattle to display.

Genetics
are the most important key to animal breeding. In fact, animal breeding is
nothing but applied genetics. Genetics and developmental biology determine
everything that is possible or not possible in breeding (leaving aside
environmental factors) and therefore it is important to look at the genetic and
developmental backgrounds of the traits that are of importance for the breeding
goals that are to be achieved. In this post, I want to give an overview on how
genetics and developmental biology influence breeding and how visible traits
come into shape. As genetics are not my major in biology (but zoology and soon
also ecology), I am glad to be corrected
if I got something wrong here in this post.

Yes,
environmental factors, but…

Of course
environment is essential for the becoming of an organism, but I will leave it
aside in this post. Surely, if animals are constantly malnourished from their
birth to death, you cannot expect them to develop in the same way as their
genetic potential enables them to. But I am going to neglect environment here,
because for this post I assume that all of the animals kept in breeding-back
are kept under appropriate, sufficient conditions, so that this factor is
eliminated (feral populations, however, might be an exception).

How genes
are inherited

It should
be common knowledge to every school kid that genes are organised on
chromosomes, and that chromosomes are the unit of inheritance. Cattle have a
diploid karyotype, at least taurine cattle have 29 chromosomal pairs. Each set
of chromosomes is inherited from one parent respectively, and passed on by
chance to the offspring. For single loci, the Mendelian rules apply. For the
whole genome, one would have to imagine a kind of bell curve. I.e. that means
that when you breed two F1 individuals made of breed A and B to each other, it
is possible that you would receive a pure individual of either breed A or B for
the second generation – but statistically most unlikely. However, it is also
usually not the case that F2 are an even mix of both parental breeds, but usually
happen to lie somewhere on the bell curve.

The goal of
breeding is to unite all desired alleles homozygous in one population and get
rid of the undesired alleles. So you have to keep in mind chromosomal genetics
when choosing animals for breeding. For details, go here or here. Chromosomes
can also put some challenges for the desired goal, f.e. if the loci of two
desired traits happen to be on the same chromosome, and breed A has the desired
allele for one locus, and breed B has the desired allele for the other. In this
case, you will never be able to unite the desired traits homozygously and
therefore stabilize both traits in the population, unless you have luck with
recombination (and then again luck in picking the right individual).

Qualitative
vs. quantitative traits

There is a
traditional difference between the so-called qualitative traits and quantitative
traits. Qualitative traits are regulated by only a handful of genes or even
just one, what makes the Mendelian rules easily discernible. A prime example
for qualitative traits are the colours of animals, which are controlled by
comparably few genes. That is why every animal breeder will confirm to you that
it is easiest to breed for colour characteristics. For example, there is the
famous Extension locus that has three
known alleles in domestic cattle: Ed,
E+ and e, in the following dominance hierarchy Ed > E+ > e. The
first allele, just to pick one, produces an excess in black pigment, and the
whole animal ends up black. If you see a completely black bull, you can be
quite sure that it has at least one Ed
allele. And so on. Therefore it is quite easy to breed for qualitative traits
as long as the loci and alleles are known (in cattle, the colour genes are
unfortunately less resolved than in other domestic animals, which is probably
because there is more interest in other domestic animals which are not
considered the mooing pre-stadium of hamburgers).

Quantitative
traits, on the other hand, are different. They are based on many different loci
which all have a cumulative or additive effect on the quantity of the trait.
Classical examples are body size, horn size and others. No matter how you
define body size (measured in length, height or weight), the number of genes
influencing it is way larger than in the case of coat colour. Human height
seems to be influenced by 54 loci according to Visscher 2008, each locus
contributing a bit to the phenotypic variance (0,3-0,5%)1. Height is
a two-dimensional measure, for total size (I assume mass) Kemper et al.
speculate that even more than 6000 genes might be involved2 (many of
them would have an only minor contribution of course). Assuming that the
genetic architecture of human height/size is comparable to cattle height/size
(not completely, of course), we can take 50 as a number to work with. If you
have such a high number of loci that have to be right, breeding takes rather
long when you have a mosaic population. So if we have a bull that has the right
colour, which involves about a dozen of loci, but not the right size, and a
bull with the right size but the wrong colour, which bull should be preferred
as sire? From the genetic perspective, definitely the bull with the right size
but wrong colour because he has 50 genes right and only about a dozen wrong,
while the bull with the wrong size has only a dozens of genes right while 50
are wrong. Therefore, the bull with the right size is about five times as
valuable from the genetic point of view (that’s why I opted for keeping
this well-built half Chianina bull in the Lippeaue herd despite its diluted
colour in 2015). Of course there are also single genes that have a dramatic
impact on body size, such as those causing achondroplasia in humans or numerous
other forms of disproportionate or proportionate dwarfisms and gigantisms in
humans and animals.

The
genetics of horns is badly researched in cattle, because there is only one big commercial
interest in cattle horns: present or not? Two loci that regulate the presence
of horns have been resolved, called Polled
and Scurred. There are no studies on
how many different loci might be involved in horn volume (I usually divide horn
size in two different factors: length and diameter), but I would be surprised
if they are less then all the colour genes in sum (for details, see this
website). So if you have a population of cattle with undesirably small horns,
it would be wiser to cross-in suitable cattle with really large horns instead
of selecting for several generations (and therefore decades) just to slowly
increase this trait, especially because other traits would get neglected. Apart
from that, there is a limit when the cumulative effects at maximum until new
mutations appear.

I was
unable to find any literature or remarks on the genetic background of horn curvature
in cattle, but the way it is inherited in heterogeneous cross populations that
show a fluent transition without following the Mendelian rules implicates to me
that it is a polygenic trait (see below).

Even though
quantitative traits are not comparable to qualitative traits in the way they
get inherited and respond to selection, and more importantly, the Mendelian
rules do not apply for the quantitative trait as a whole, the individual
chromosomes with the involved loci do follow the Mendelian rules of course.
This thought was the basis when I set up my “F”-based breeding plan.

The role of
individual genes

The old
one-gene-one-trait axiom is not used in modern genetics anymore, and it also
creates a wrong impression. Genes do not work parallel and un-affected from
each other, and it is not just one gene that creates one trait. Many genes actually
are responsible for more than one trait, called pleiotropy. Selecting on
pleiotropic genes will therefore also have an effect on other traits, what
happened in domestication. Colour genes, for example, a mutation on the Agouti locus in mice (which regulates
the expression of red pigment in mammals) is found to be associated with
obesity3. A deviant brown colour variant found in some Lippeaue
Taurus cows seems to be associated with brittle horns and fur, therefore maybe
correlated with some metabolic disease as I was told (see here). For details go
here: The Dedomestication Series.

The
reverse, a trait that is caused by many genes, called polygenic trait, is no
rarity either. Most traits are actually more or less polygenic. Furthermore,
genes also influence the expression of each other. When a gene inhibits the
expression of other genes on other loci, it is called an epistatic gene.

How an
organism takes shape

It is basic
biological knowledge that (most) biological cells carry (almost) all of the
genetic material of an organism. However, if from the start of fertilization
all cells would start to express all genes, an amorphous mass of cells and not
a viable organism would be the result. Therefore, the genetic plan needs a plan
itself how to be expressed properly so that a living, differentiated organism
takes shape. This role is taken by epigenetics on the one side and regulator
genes on the other side that determine the timing and where genes are
expressed. Genes regulate each other; how long, where and how much gene product
is produced. I do not want to go much more into details of developmental
biology here, but if you are interested in this subject I suggest you to dig
into the literature on masterregulator genes, the genetic toolkit, EvoDevo etc.
– it is a fascinating subject.

Transcription
factors regulate how long, where and how much of genes are expressed, and this
developmental regulation plays a crucial rule in how an organism will be shaped
in the end. Different phenotypes can be produced with the same genetic
background just by extending or shortening the expression of certain genes. A
typical example is developmental delay as a result of domestication: the
development stops earlier than in the wild type, and as a result, adult animals
retain juvenile conditions f.e. in skull anatomy or behaviour (called
paedomorphy or neoteny). For more details, see the Dedomestication Series. Many
developmental genes work by the feedback loop principle: if the level of a
signal molecule is high enough, the gene starts to express its product, if it
is below, the gene will not be produced. Hormones, as signal molecules, play a
significant rule in the developmental cascades that shape an organism. Hormonal
disorders and castration are known to have dramatic physiological impact that
also manifests in an altered morphology, and many of the morphological changes
we see in domesticated animals are seemingly linked to developmental changes
caused by hormonal pathways (again, see the Dedomestication Series).

Horn
length, curvature and size are probably also influenced by developmental
factors. It is logical: if the transcription factors that say it is time to
grow horns are never produced, horns are not going to be produced. If the
transcription factors never stop initiating expression, horns will continue to
grow. If the development of the horn curve is stopped earlier than in the wild
type, it will never reach its full shape. I came up with this idea when playing
around with the horn sheaths of the skull of the Taurus bull Latino, and
discovered that when pushing the sheaths towards almost the end of the bony
core while following its curve, and therefore imitating continuing growth of
the horn in length, suddenly the banana-shaped horns of Latino turn into the
curled horns of an adult aurochs bull that even have the right size. I made two
sketches illustrating that interesting observation.

Left: the skull with its horn sheaths in place. Right: horn sheaths moved outwards following the curvature, and an
aurochs-like horn shape and length appears

Based on
that, I inferred that perhaps if the development of the horn growth was not
stopped prematurely but transcription factors would have continued to induce
horn length growing following the – only seemingly weak – curvature, the horns
would have developed the curvature and length of that of an aurochs. If this
idea is correct, it would also imply that this Sayaguesa x (Heck x Lidia)
crossbreed has the right genetic make-up for horn curvature and length, and
that developmental factors are the reason why the horns do not look like those
of the wild type. This would then apply also to a lot of other domestic bulls
and also cows.

[I could
actually imagine that the genetic architecture of the aurochs horn curvature is
not that complex – perhaps it is only one protein produced by only one locus
that causes the horn to curve in a spiral, the so-called primigenius spiral, instead
of growing straight out of the skull like a pen. There could be several
mutations on this locus and also several epistatic genes to this gene that
causes the variety of horn shapes we see in domestic cattle. But as long as
nobody who has the possibility to get the funding is interested in studying
this question, this resembles speculation.]

The most
interesting note in this respect is that steers show a totally different
morphology than bulls, but with the same genetic make-up. Steers tend to grow
larger, have longer legs, horns and snouts and therefore, are superficially
more aurochs-like in these respects. This is probably because the removal of
the gonads alters the timing of the development. The gonads do not stop the
production of certain transcription factors because they are absent and
therefore, the development gets extended – the opposite of what has happened in
domestication, and as a result, steers are superficially more aurochs-like in
certain morphological respects than functional bulls of the same genotype.

Holstein steer displaying inwards-facing horns, much more so than any functional bull of this breed

Very high-legged Hungarian Grey steer; functional bulls of the same breeds are built way more longish

Sexual
dimorphism

Cattle have
the XX:XY gonosome system and therefore have the easy standard mammal scheme
(there are also mammals that lack Y chromosoma, or have multiple X). Basically,
all mammal embryos start as females and then the male-specific genes on the
male-specific regions on the Y chromosome start to work that are responsible
for the differences between male and female. That is not to say that male
traits are found exclusively on the Y chromosome. For a lot of traits, the
blueprint is on the autosomes, and the Y chromosomes just provides the switch
to make them work, such as a high testosterone level. Sexual hormones,
especially steroid hormones, are the most important factors in sexual
differentiation and not surprisingly they are mostly produced in the gonads but
also in female kidneys. For example, the amount of melanisation on E+ cattle is said to be regulated by the
testosterone level, therefore most bulls of wildtype coloured cattle are darker
than the cows. But cows are capable of expressing a “bull colour”, as we see in
many breeds. In the aurochs, this difference in testosterone level was much
more well-expressed, in colour as much as in morphology and size. Domestication
reduces sexual dimorphism. This was probably not achieved by actively selecting
against sexually dimorphic traits, but was the results of pleiotropic effects
and developmental cascades as the result of selection on tameness alone, as the
farm fox experiment suggests (again, for more on that see the Dedomestication
series).

As for
selecting on sexually dimorphic traits, I think the conventional method won’t
work here. Always simply choosing red cows and dark bulls, or small cows and large
bulls, or whatever sexually dimorphic trait, probably will work not here. You
would actually have to choose individuals where the sexual dimorphism is laid
down in the genome. A phenotypically red cow might either have a more or
less strong sexual dichromatism (s. dimorphism in colour), or simply be red and
have no sexual dichromatism at all, depending on the bulls that she would
produce. The same is the case with dark bulls. How to know that? Well, at first
the genetic background of the dimorphic trait has to be resolved, and then the
individuals would have to be tested. But since this is rather impractical on a
large scale, one could also look at the ancestors, siblings and offspring of
the individual in question. But for that, you would have to have a significant
number of siblings and offspring to get an idea of the genotype, and what makes
it especially challenging is that crossbreeds are genetic patchworks that are
rather hard to judge.

A good
example why the conventional method of simply always taking red cows and dark
bulls is not a proper way to achieve true sexual dimorphism are the Sayaguesa
of Peter van Geneijgen. Sayaguesa usually have very reduced sexual dimorphism:
bulls are black, cows either bull-coloured or very dark coloured. This
particular Dutch herd is slightly influenced by Alistana-Sanabresa, a related
and lighter-coloured breed with red cows and bulls with a colour saddle. Some
of these Sayaguesa, as a consequence, show the reddish brown coat colour desired
for aurochs-like cows. I was happy at first. However, as a consequence, some
Sayaguesa bulls started to show a colour saddle. This implies that the sexual
dichromatism did not truly increase, the change was merely cosmetic as there is
now more variation in this respect.

Therefore,
I think there are only two possibilities regarding breeding a good sexual
dimorphism: either relying heavily on a breed where a well-marked sexual
dimorphism is still retained (such as Maronesa, for example), or putting up
with the fact that there will always be a certain number of cow-coloured bulls
and bull-coloured cows (the latter are, however, historically confirmed to have
existed in aurochs populations on occasion) and that the size difference
between the sexes is not that big as in the aurochs.

Implications
for “breeding-back”

Now I am
going to give a quick summary of what is written above on which traits relevant
for “breeding-back” belong to which category.

As I wrote
in the beginning, breeding for qualitative traits, and therefore colour
characteristics is comparably easy. However, it is no secret that recessive
genes are rather tricky to breed out. Especially because it becomes more
difficult the less frequent a recessive allele is, because it shows up less
frequently. The most effective way to clear a recessive allele from a
population is to genetically screen all the individuals for it, and take all the
carriers out of the population, but that would be costly. But it is not
impossible to get rid of recessive alleles on the conventional way; in Heck
cattle, it seems that the grey Agouti
dilutions inherited from Steppe cattle have been purged out in the Neandertal-Wörth
lineage at least.

For
quantitative traits, it is probably best to try to compensate the extremes. If
you work with a number of breeds that have too small horns, using at least one
that has very large or “too large” horns would be advantageous, as the results
will have intermediate horns and you would have to purge out offspring showing
the extremes in the future. Otherwise you would have to start a long phase of
selecting only for the quantity of that one trait and neglect all the other traits,
which probably not practical as “breeding-back” focuses on so many phenotypic
traits.

(That is
why I worry a bit about the average body size and horn size of the cattle used
in the Tauros Project, but it is probably too early to judge the situation in
this case)

The most
complicated traits to breed for are those that are influenced by development in
the list above. Domestication dramatically altered the developmental biology of
cattle, resulting in the gross morphological differences between domestic
cattle and aurochs. How can we revert this by selective breeding? Is it
possible at all? One could say that it is of course possible to select on all
traits that are quantifiable. However, fur breeders tried unsuccessfully to breed
foxes for earlier maturity by selecting on this trait directly. In contrast,
selecting on tameness resulted in foxes reaching earlier maturity as they
slowly became more and more domestic4. It were pleiotropic effects
that caused developmental changes to reach exactly that. And most importantly, we
do not know all the involved gene loci for those developmental mechanisms, and
do not know the developmental cascades in particular. In turn, I think it will
not work to achieve long, aurochs-like snouts if you work with a population that
exclusively has shortened snouts, even if it shows a bit of variation in snout
length. As with sexual dimorphism, it would probably be necessary to get
individuals into the mix that show the adult, aurochs-like skull shape that is
desired because there obviously the developmental processes are still like in
the wild-type in this respect. Luckily, there are a number of cattle on this
world where we can find an aurochs-like skull shape, including primitive breeds
and also derived ones (some derived breeds, like Holstein, even more so than
most primitive breeds). The same also goes for body shape and other
morphological factors. There are some breeds, luckily, that still show a rather
aurochs-like body shape in having a muscular body with a slender waist resembling
wild bovines. These include Lidia, Corriente and Camargue (and it is probably
not a coincidence that these breeds are not bred for docility and mass, but for
agility and “fighting spirit”). As with sexual dimorphism, it might be useful
to rely heavily on those breeds where the developmental set is obviously still
right to produce an aurochs-like body shape.

Even more
efficient to get rid of typical domestic traits in the morphology of cattle
might be to do the exact reverse of domestication: to breed with animals that
mature later and select on a slow individual development, and to also select on
“wild” behaviour (note: not necessarily aggressive). Breeding for an extended
development might really work wonders for achieving a wild-type morphology. But
the problem is that it will take really, really long (not to mention that the
breeding itself takes longer then). Perhaps a century and more. We cannot
effort endless patience since there currently is a time frame where a lot of
suited areas become free for extensive grazing and the reintroduction of large
herbivores, therefore we have to find a compromise between breeding the
“perfect aurochs mimic” and taking the chances of filling reserves with cattle
that are fit for the job. Of course selective breeding and releasing the cattle
in large nature reserves do not exclude each other, but to the point where the
cattle get to live more naturally and less controlled, the breeding influence
of man is becoming less. The second aspect, breeding the cattle for wild
behaviour and hoping that the same pleiotropic effects will reverse domestic
traits that caused them, is even more impractical. Actually, extremely
impractical. Let us be honest, nobody, really nobody, who has to work with the
cattle wants them to behave like a wild animal. It is dangerous, costly,
exhausting, and risky for the project, people and cattle. It is surely not a
coincidence that most grazing projects work with cattle instead of wisent (and
surely not a matter of the habitat), and if aurochs could finally be recreated
genetically, they would probably not be used as often as cattle either.

What does
this all mean for breeding-back? It is probably not all that problematic to
breed an accurate aurochs-like colour (as some Heck herds have shown), with
recessive variants showing up occasionally for quite some time. It is also well
possible to breed cattle with the right body and horn size. As for the horn
curvature, patience is needed, as truly aurochs-shaped horns are not that
frequent among modern cattle, even in primitive breeds and breeding-back
populations. For the proportions, and to a lesser extent also the body shape,
there are a number of breeds that are very suitable, and some of these are
currently used in breeding-back. The same goes for the skull shape. But I
assume that, no matter which project or breed in particular, that horn shape,
body shape and proportions will not end up perfectly aurochs-like and
universally distributed among the individuals of the populations in the
breeding-back results except after a very long period of selection because of
the complicated genetic and developmental background of these traits and the
very large number of genes involved. As for sexual dimorphism, I assume that
the degree of sexual dimorphism will always remain below the extent of the
aurochs for the reasons outlined above.

Selective
breeding alone, especially when many of the key genes (developmental genes) are
not known or visible, will probably not enable us to fully reverse
domestication and turn domestic cattle into a morphological wild type. And we
also have to assume that a lot of the specific gene material of the aurochs was
lost during domestication. However, we are probably able to mimic it to a large
degree, and then releasing the product into nature and letting natural
selection work will probably retrieve or refine a lot of wild type
characteristics. For more on that, see the Dedomestication Series.

5 comments:

"As for selecting on sexually dimorphic traits, I think the conventional method won’t work here. Always simply choosing red cows and dark bulls, or small cows and large bulls, or whatever sexually dimorphic trait, probably will work not here."So is it that both cows and bulls can pass on the sexual dimorphism, are they weighted equally ? Or may it be mostly the bulls that carry pass these on, could it be bound to the Y-chromosomes ? In the first case you would be rigth, otherwise it could work for cows...but one should be more picky with the bulls. Then the cows should be red, and the bull-lines should be from breeds with cows that show little or no black color.

"These include Lidia, Corriente and Camargue (and it is probably not a coincidence that these breeds are not bred for docility and mass, but for agility and “fighting spirit”). As with sexual dimorphism, it might be useful to rely heavily on those breeds where the developmental set is obviously still right to produce an aurochs-like body shape."Is sexual dimorphism especially high in this breeds ? Or could the cows here have a larger influence on the appereance of their male offsprings ? This breeds also have athletic cows, so maybe it's just that the bulls look auroch-like because the extra-muscles are build on the right frames ?

To your first question: No, both males and females pass on dimorphic traits and also the extent of it. Of course in cattle a Y chromosome (or the necessary signal molecules) are needed to express male traits, but the traits themselves and the code for the intensity of these differences can also be found on the X chromosome or autosomes. So both cows and bulls pass on dimorphic traits. To your second question: If you are talking about sexual dichromatism, in Lidia the situation is often unclear because there are many lightly coloured bulls and dark cows, but there seem to be some herds where the dimorphism is ok but not perfect (so basically similar to what we see in Heck cattle). Corriente seems to be quite good regarding sexual dichromatism based on what I have seen so far. In Camargue, the sexual dichromatism cannot be judged because it is masked under the black colour allele E^d. The only aurochs-like breed that I am aware of that almost always has a clearly marked sexual dimorphism is Maronesa; to a varying extent.

Thanks for reply.Interesting.You mean Maronesa is the only breed with a clearly expressed sexual diChromatism, don't you ?From the Lidia-strains at least some individuals of the the Casta Navarra have some degree of dichromatism. This breed is on the smaller side, but maybe due to the colour it would be less tricky to cross with this than with Camargue...

To that XY-dimorhism-question, may it be that some breeds are better in passing on the male part of it than others ? Could it make sense to focus on certain bull-lines, Maremmana and Pajuna for example ? Because there is at least one very good cross of these...

The black allele E^d carried by Camargue is not problematic, as it is dominant and therefore easy to select out. I would not worry about that.

Why should some breeds be "better at passing on the male part"? What's that supposed to mean, are you talking about the Y chromosome? I think the looks of Manolo Uno are ok, but the looks of an F1 are completely irrelevant as it is solely the product of heterozygosity - it does not tell you what to expect for later generations (except for dominant-recessive traits). F1 do not give an accurate idea of the potential for a breed combination. So if you cross two breeds and the result seems to be disappointing in F1, it does not mean anything for further breeding.

Well, just wanted to say that in Navarra the dichromatism is already unmasked...Yes, i think i basically mean the Y-chromosome. If this is the case maybe it could make a breeding-strategy to build two pools, and finally take only the cows from the one and the bulls from the other and mix these up.

Well, if it would be completly unpredictable it would hardly be possible to make any decisions about selection.I think if two breeds are combined that are much different then one could expect more surprises in future generations, and if there are certain similarities then it's likely that these could be kept up.So in general i would expect that the result of an Pajuna-Maremmana-cross is more predictable than the result of an Watussi-Chianina-cross, for example.(my comments tend to disappear, is thias due to moderation ?)

About this blog

This blog is on everything related to the so-called “breeding-back” of extinct animals: From the extinct animals themselves, over their often domestic descendants and dedomestication to news and facts about various breeding-back projects, reports and photos from my own breeding-back related trips. I try to have a balanced and fact-based approach to this subject and to dismantle many of the popular myths. Enjoy!

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About me

My major interest always have been extinct animals, from dinosaurs to Pleistocene megafauna and more recent extinctions. Besides that I am interested in evolution, genetics and ecology.
I am also an amateur animal artist, making drawings and models mostly of extinct animals.